Despite considerable advances toward understanding the molecular basis of calcium and phosphorus transfer across the intestine and other epithelial membranes, much is yet to be revealed. This also pertains to the molecular basis of vitamin D action. The proposed investigations represent a continuing effort to uncover new aspects of the problem and to aid in defining the mechanisms involved, which should find relevance to specific human and animal disease states. Particular attention is to be given to the vitamin D-dependent calcium-binding protein (CaBP), its properties, ultrastructural localization, role in calcium translocation, and relation to other vit. D-dependent factors. Regulatory mechanisms will continue to be investigated, as will the use of CaBP as a sensitive indicator of vit. D activity. The organ culture system with embryonic intestine, which responds to vitamin D and metabolites, constitutes a unique tool for the investigation of specific aspects of the vit. D- dependent absorptive process. Supportive evidence for a shunt path for calcium across the intestine (and possibly kidney) will be sought. Detailed investigations on the physiological and molecular basis of phosphate absorption are planned. The potential applications of these findings to pertinent disease states in humans and animals will be continually surveyed.